Four years ago, the fossil-fuel lobby helped to beat back President Obama's proposed cap-and-trade system to reduce greenhouse emissions. Exxon Mobil, America's most profitable company, led the way in global warming denial. Times have changed and Exxon Mobil is beginning to sound more like Al Gore. The oil giant would even be willing to pay a tax on carbon emissions. Other oil companies would too, along with Walmart, DuPont and General Electric. They're now in direct opposition to former Republican and Tea Party allies, at a time of warnings that climate disasters could be "sudden and unexpected." Would a carbon tax help to slow global warming? What would it mean for company profits and the cost of living?